Because black flies (Simuliidae) and mosquitoes (Culicidae) are major vectors of human parasites (particularly in tropical areas), the development of effective control measures is of great importance. This proposal is designed to evaluate the potential of two pathogens - a nematode and a bacterium - for the biological control of these insects. The nematode Neoaplectana sp./Strian DD-136, although not recorded as a natural parasite of black flies, has in our preliminary tests proved capable of causing high mortality (70-100 percent) among black flies, particularly late instars. These filter feeding larvae readily ingest the nematodes from the passing streamwater and die generally within one week postexposure. In the proposed research, initial laboratory tests will determine the significance of host, parasite and environmental variables in achieving high rates of black fly mortality. The susceptibility of non-targets and the potential for establishment of the nematode in treated areas are also primary objectives and will be thoroughly examined in a comprehensive series of laboratory bioassays. The field effectiveness of the nematode will be subsequently determined in a series of small-scale stream trials. The bacterium Bacillus sphaericus is currently regarded as one of the most promising agents for mosquito biocontrol. Laboratory tests have demonstrated this bacterium to be toxic to a large number of mosquito species and to have little if any effect on non-targets. The present proposal is designed to determine the effectiveness of the bacterium when subjected to a wide variety of experimental conditions. This will initially be accomplished by comprehensive laboratory bioassays, with subsequent introduction of the pathogen into the field environment.